System and method for laser ultrasonic bond integrity evaluation

ABSTRACT

A nondestructive bond testing system is implemented using a pulse laser that sends a single or multiple pulse(s) of controlled magnitude and bombards an object of interest causing a thermoelastic excitation response. This excitation in turn induces an ultrasonic propagation along or through the surface material. By detecting, capturing and interpreting these thermoelastic propagation signatures, the attachment condition of the joining materials is determined. The technique is a significant improvement over traditional mechanical pull, shear or contact type techniques. The techniques are implemented in automated high speed inspection systems suitable for real time manufacturing application. Particular applications include evaluating material joining in microelectronics manufacture (such as ball bonds) and thin coating processes.

RELATED APPLICATIONS

This application is a continuation of prior-filed non-provisionalapplication Ser. No. 09/215,374 of the same title filed Dec. 18, 1998,now U.S. Pat. No. 6,181,431 said application being hereby incorporatedby reference as if fully set forth herein. Said prior application inturn claims benefit of prior-filed provisional Application No.60/068,362 filed Dec. 19, 1997, said provisional application beinghereby incorporated by reference as if fully set forth herein.

GOVERNMENT RIGHTS

The U.S. Government has limited rights in this invention pursuant tocontract No. N66001-95-C-7021 between the United States Navy and SimpexTechnologies, Inc.

FIELD OF THE INVENTION

The present invention relates generally to an apparatus and method fordetermining bond integrity (adhesion status) of joined materials at amicro-level.

BACKGROUND OF THE INVENTION

There has been a long-felt need for non-destructive inspection methodsthat can be used to determine the integrity of a bond between two items.This need has been felt particularly in the case of bonds formed at themicro-level. The micro-level can be defined as involving at least onematerial with a dimension on the order of 0.005 inch or smaller. Suchinspection methods might be applied to testing ball and wedge bonds,thin coatings, circuit traces, ribbon bonds, solder balls, surface mountcomponents, PIN grid arrays, and MIMMs commonly used in microelectronicsinterconnects. The materials joined in these applications include, butare not limited to, silicon, silicon carbide, aluminum, gold, galliumarsenide and the like.

For example, ball bonds used in connecting silicon wafers to externalcircuits through very fine wires are typically tested, if at all,according to military specifications which require a pull test of eachbond. This test is performed using a machine which sequentially hookseach wire and applies a predetermined pulling force to determine whetherthe associated bond will hold. This technique has significantlimitations. In particular, the inventor has discovered that if thistest is performed repeatedly on the wires of the same device, anincreasing number of wires typically pull loose with each succeedingtest. This result implies that the test does not truly qualify asnon-destructive. That is, each application of a pulling force to a wireweakens its bond and repeated testing will actually break the bonds. Itis possible that a bond might pass a single pull test of this type, butthat the test would leave the bond precariously connected and destinedfor failure in the field when subjected to ambient vibration, shock, ortemperature variations.

In the case of bonds having larger dimensions, such as pipe seams, weldsused in automotive and marine manufacturing, etc. various x-ray andacoustic techniques have been applied to analyze the condition of aninterface between two items. Laser ultrasound techniques have also beenproposed. For example, U.S. Pat. No. 4,659,224 and U.S. Pat. No.4,966,459 to Monchalin, U.S. Pat. No. 5,081,491, to Monchalin et al.,and U.S. Pat. No. 5,137,361 to Heon et al. disclose the results of earlyresearch in this field. U.S. Pat. No. 5,103,676 to Garcia et al. shows afurther method of laser ultrasonic process monitoring.

Laser techniques have also been considered for use with smaller bondssuch as those found in semiconductor circuits. U.S. Pat. No. 5,201,841to Lebeau et al. proposes a thermal gradient technique, and JapanesePatent Publication 62-7198 Jan. 14, 1987 by Hitachi Research Corp.appears to propose a laser technique.

The inventor's prior U.S. Pat. Nos. 5,420,689, 5,424,838, and 5,302,836disclose lighting methods and apparatus useful in small-scale laserultrasonic measurement. U.S. Pat. No. 5,535,006 to Siu et al. builds onthe inventor's earlier work and discloses a method of evaluatingintegrity of adherence of a conductor bond to a substrate.

However, as far as the inventor is aware, none of these prior systemshas provided an effective alternative to pull testing of wire bonds, oran effective method for analyzing thin film coating integrity at themicro-level. The results produced by the systems disclosed in theinventor's own prior patents, while encouraging, were not consistentenough for industrial application.

Thus, there is a need in industry for improved methods and systems ofthis type that will provide repeatable, accurate, and trulynon-destructive testing capability.

SUMMARY

Therefore, it is a general object of the invention to provide animproved system and method for determining the bond integrity (adhesion)status of adjoining materials using laser ultrasonic techniques.

It is another general object of the invention to provide an improvedsystem and method for determining the bond integrity (adhesion) statusof adjoining materials using a pulse laser which applies heat onto thesurface of interest, resulting in generation of a thermoelasticpropagation (surface, bulk, air waves or combination thereof) in alldirections from the pulse point, which can be detected using astabilized continuous wave laser using interferometric techniques.

Another broad object of the invention is to provide an improved systemand method that is particularly adapted to nondestructively test andevaluate the thickness and/or uniformity and adhesion characteristics ofa thin coating of material applied to a substrate.

Another general object of the invention is to provide an improved systemand method that is particularly adapted to nondestructively determinethe bond integrity of joining materials at the micro-level, such asmicroelectronic interconnects, ball bonds, wedge bonds, circuit traces,surface mount components and MIMMs.

It is also an important object of the invention to provide an improvedsystem and method for analyzing thermoelastic propagation signatures,single or in combination, to interpret bond integrity (adhesion) ofadjoining materials, and determine whether they are fully bonded,partially bonded and touching yet non-bonded.

Another useful object of the invention is to provide a fully automatedbond integrity determination system with particular applications ininspection and testing in microelectronics manufacturing processes.

An additional object of the invention is to provide improved operationaltiming and signature gathering methods and apparatus for use in a laserultrasonic measuring system.

It is also an object of the invention to provide a laser ultrasonicmeasuring system with a vastly improved signal to noise ratio fordetected wave propagation signatures.

A further object of the invention is to provide a method for correlatingsurface wave propagation to bond integrity in the context of a bondtesting system.

These objects and others are achieved, in a preferred embodiment of thepresent invention, by providing a pulse laser and a continuous laserdetector forming a cause and effect sensing device. The pulse lasersends a single or multiple pulse(s) of controlled magnitude and bombardsthe object of interest causing a thermoelastic excitation response. Thisexcitation in turn induces an ultrasonic propagation along or throughthe surface material. By detecting, capturing and interpreting thesethermoelastic propagation signatures, the attachment condition of thejoining materials is determined. The technique is a significantimprovement over traditional mechanical pull, shear or contact typetechniques. The object need not be contacted by mechanical means, theexcitation is much gentler than that required in a contact test, and thespeed of the test is much faster than other automated manufacturingprocess, making it suitable for real-time process control purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a microelectronic ball bond showing the applicationof a pulse laser and detection laser for signature analysis.

FIG. 2 is a schematic diagram of an optical system for evaluating bondintegrity according to the present invention.

FIG. 3 is an illustration of a ball bond which has an ablated surfacedue to highly localized laser power application.

FIG. 4a is a view showing the application of a pulse laser beam spotmuch larger than a ball bond to be tested, and FIG. 4b is a graphshowing the amplitudes of a resulting surface wave and shock wave,respectively.

FIG. 5 is an end view of a preferred embodiment of a power controlassembly according to the invention;

FIG. 6 is a side view of a pulse laser calibration and synchronizationapparatus according to the invention.

FIG. 7 is a timing chart showing the effect of a variable delay incomponent response to a command to activate the pulse laser and datacapture on the identification and collection of data. The variation inresponse time is very large compared to the window during which usefuldata may be collected, making it difficult to select that window.

FIG. 8 is an optical schematic diagram showing details of the detectorlaser transmission and reflection detection optics.

FIGS. 9a and 9 b are side views showing an alternative embodiment of theinvention wherein bond integrity is detected by applying a laser pulseadjacent to a bond and detecting the resulting surface wave on thesubstrate on the other side of the bond.

FIG. 10 is a top view of a ball bond showing another alternativeembodiment in which the intermetallic structure of a bond is analyzed bycollecting vibration propagation data at a plurality of points aroundthe bond.

FIG. 11 is a diagram showing the operation of the invention's staticmode for setting sensitivity of the interferometric circuit.

FIG. 12 is a diagram showing the operation of the invention's dynamicmode for setting the sensitivity of the interferometric circuit.

FIG. 13 is a graph comparing responsivity of a silicon PIN photodetectorto a Gallium Arsenide PIN photodetector for various wavelengths.

FIG. 14 is a graph showing sample bond integrity signatures fornon-bonded, partially bonded, and fully bonded cases respectively.

FIG. 15 is a simplified bond integrity signature diagram showing keyanalysis points for the waveform.

FIG. 16 is a schematic diagram of a preferred embodiment of thesignature analysis system according to the invention.

FIG. 17 is a flowchart showing the operation of an automated bondintegrity testing process.

FIG. 18 is a top view showing energy dispersion of single pulse laserand detector laser footprints.

FIG. 19 is a top view showing energy dispersion of a wide or multiplepulse laser footprint used in an alternative embodiment.

FIG. 20 is a side sectional view showing the application of the laserultrasonic techniques of the present invention to analysis of a thincoating.

FIG. 21 is an exemplary graph of Rayleigh wave velocity versus frequencyfor various thicknesses of a thin coating of aluminum on a siliconsubstrate.

FIG. 22 is a top view of an alternative embodiment of the inventionwhich uses a curved pulse laser spot to create surface waves focused ona single detection point.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described first in terms of an embodimentwhich is particularly useful in a variety of small scale bond testingapplications. Such inspection methods may be applied to testing thebonding integrity of any materials at the micro level. The bonds testedmay include, but are not limited to, ball and wedge bonds, thincoatings, circuit traces, ribbon bonds, welds, solder balls, surfacemount components, PIN grid arrays, MIMMs, and various types of adhesionmedia and methods commonly used in microelectronics interconnects. Thematerials joined in these applications include, but are not limited to,silicon, silicon carbide, aluminum, gold, gallium arsenide and the like.

One such application is the testing of the connection of amicroelectronic ball bond or other small bond to its bonding pad, andthe first embodiment of the invention will be described using thisapplication as an example. FIG. 1 shows substrate 102, on which bond pad104 is mounted. A bond wire 106 is connected via ball bond 108 to bondpad 104. A pulse laser beam 110 is applied to ball bond 108, while thereflection of detector laser beam 112 is used to detect vibrationspropagating through substrate 102 as a result of the application ofpulse laser beam 110.

The two laser beams are applied through a complex optical system whichis shown in schematic form in FIG. 2. This optical system has two mainobjectives. One is to direct and focus the pulse laser beam 110 to theintended target such as ball bonds, wedge bonds or interconnects. Thesecond objective is to direct and focus the detector laser beam 112 topick up the propagation waves generated by the pulse laser beam 110.

As illustrated in FIG. 2, the optical system of the preferred embodimentincludes pulse laser 1, mirror 2, beam expander 3, power adjustment 4,beam splitter 22, synchronization photo detector 5, laser reflector 23,focus lens 24, density filter 25, video camera 6, laser reflector 20,objective lens 7, part table 21, quarter wave plate 12, polarized beamsplitter 11, half wave plate 10, beam expander 9, detector laser 8,cutoff filter 19, polarized beam splitter 13, focus lens 17, photodetector 18, quarter wave plate 14, interferometer 15, and photodetector16. The function of each of the components will now be explained indetail.

Pulse laser 1 is preferably a 1064 nm wavelength pulse laser with 10nsec. pulse width (pulse duration) and pulse repetition rate of 100 Hz(100 times per second), is capable of delivering, for example, one Wattof energy. Since the wavelength of this pulse laser is the same as thedetector laser, a frequency doubler was used to convert its useablewavelength to 532 nm and power to ½ watt. One appropriate laser is aModel #S10-5230 manufactured by Spectra-Physics. Other lasers could beused as long as they provide a similar high repetitive rate, short pulseduration, and consistency of pulse to pulse power levels. The lasershould be selected to have a relatively low power because most higherpowered lasers do not have high resolution in power control adjustment.

The pulse laser frequency preferably does not have the same frequency asthe detector laser, so that the dispersed light is not interpreted bythe photodetectors as false surface waves. The pulse width of the pulseis preferably short, such that the pulse is not continuing while thesurface wave is already arriving at the detection point—especially whenthe distance between the pulse laser and the detector laser is in closeproximity to each other and that the Rayleigh velocity for that materialis fast—such as silicon etc. Ten nanoseconds is an appropriate pulseduration in typical micro-level applications. The pulse width of thepulse laser may be varied, thus changing the rate at which heat isapplied to the pulsed surface. However, such variation typically changesthe shape of the surface wave signature.

Surface mirror 2 is a surface reflector which receive the output ofpulse laser 1 and redirects the pulse laser beam into beam expander 3.Of course, surface mirror 2 may be omitted if the laser is mountedhorizontally.

Beam expander 3 controls the collimation or divergence of the laser beamas it exits the pulse laser, to produce the desired beam (spot) size ofthe pulse laser when it reaches the point of interest (i.e. the ballbond, wedge bond and any other interconnects). The inventor has foundthat it is important to control the spot size of the pulse laser suchthat is appropriate to the size of the object being pulsed. In the caseof a ball bond, the spot size of the pulse laser should be approximatelythe same size as the ball bond (0.003″ in diameter), and preferablyslightly larger than the ball bond. If the spot size is too small, thepower density of the laser will ablate the surface materials. Thisablation effect (vaporization of materials) will deposit undesirablemetallic debris on the microcircuit. FIG. 3 shows a ball bond 108 withan ablated region 302 resulting from application of a highly focusedlaser pulse that is much smaller than the ball bond. Reducing the laserpower will eliminate the ablation effect. However, reducing power alsohas the undesired effect of reducing the magnitude of the surfacewavesignature which must be detected. Therefore, a balance between thepulse laser power and the spot size of the laser important foroptimizing signature and minimizing any damaging effect on the materialsbeing pulsed.

Preferably the size of the pulse laser spot is slightly larger than theball bond, and the laser power is adjusted so that no ablation occurseither on the ball bond or on the substrate areas adjacent to the ballbond. The possibility of increasing the pulse laser spot to a size muchlarger than the ball bond was also considered. While this approach maybe used, it is less preferable because it has potential undesired sideeffects. In the illustration of FIG. 4, the pulse laser spot is muchlarger than the diameter of the ball bond. A significant amount of laserenergy (1) “spills” over ball bond and is pulsing the surface to whichthe ball is bonded. This may damage the surface materials because theynormally do not have as high a melting point as gold, thus ablation ofmaterial occurs. In addition, a surface wave is generated from thesurface and will arrive at the detection point first. As long as thereis no ablation of the surface, these two separate waves will arrive atdifferent times—one from the surface (A) and the other from the top ofthe ball bond (B). This will still provide adequate distance between thetwo signatures for analysis. However, if ablation occurs on thesubstrate surface, the shock wave from the surface (A) will arriveapproximately the same time as the wave from the ball bond (B), thuscausing an overlapping of signatures. This invalidates the detection ofthe signature of interest resulting from application of heat to the topof the ball bond. Also, if the pulse spot size is excessively large,this will inhibit placement of the detection laser close to the pulselaser, and the increased distance between the pulse and detector laserwill provide a weaker signature or poorer signal-to-noise ratio. Theinventor's experience suggests that for microelectronics ball bonds, thespot size should typically be between 0.001″ to 0.005″ in diameterdepending on the diameter of the ball bonds.

FIG. 5 shows the structure of a preferred embodiment of high resolutionpower control assembly 4, which is a means for finely adjusting thepower transmitted by the pulse laser to the bonded item. This embodimentof high resolution power control assembly 4 comprises a half wave plateassembly 502, micrometer adjustment 504, a polarizing beam splitter cube22, and a power trap 26 (shown in FIG. 2). Half wave plate assembly 502comprises frame structure 505 which supports wave plate 506 via bearings508. A spring 510 biases a lever 514 (attached to wave plate 506)against the screw 512 of micrometer 504. The function of assembly 4 isto provide much finer power control of the pulse laser than is providedby the power controls of a typical laser panel. As discussed above, theamount of power from the pulse laser must be well controlled to preventmaterial ablation. In this design, wave plate 506 may be rotatedprecisely in both clockwise and counter clockwise directions toestablish a desired position, by adjusting the high precisionmicrometer. The rotational position of the wave plate will establish apolarization of the laser light which will interact with thepolarization of beam splitter 22 (shown in FIG. 2) so that a variablepercentage of the laser light power is passed through both wave plate506 and beamsplitter 22. A 90 degree rotation of the wave plate willpermit a full range of power (0 to 100%) to pass through. The use of themicrometer provides an infinite degree of power level control for thesystem. Due to the fine adjustment possible with micrometer 504 and theadvantage provided by lever 514, it is possible to select a rotationalposition of wave plate 506, and thus a power throughput level, withgreat precision. This power control design can be used to accommodatedifferent materials having different ablation tolerances. Laser trap 26(shown in FIG. 2) is used to capture the residue laser power for safetyreasons.

In addition to tight power control, a method for monitoring andcalibrating pulse power during operation is also necessary. Systemsynchronization and power calibration assembly 5 is preferably mountedin line with the pulse laser as illustrated in FIG. 6. A small piece ofthin glass 602 (medical grade) is attached to the end of monitoringtubes 604 at an angle (45°) as illustrated in FIG. 6. As the laser pulse110 passes through the thin glass 602, a very small amount of laserlight is reflected 90° upward, while most of the light energy penetratesthrough glass 602. A high speed photodiode 606 is mounted permanently onthe first monitoring window 608 to capture part of that reflected laserenergy from the laser flash. The presence of this laser flash confirmsto the system that the laser has indeed fired and that the laser pulseis on its way to the test surface. This confirmation flash initiatestime (0) for the high speed transient recorder in a manner which will bedescribed later in more detail. The second window 610 is for attaching acommercially available power meter 612 to perform periodic adjustment,calibration and verification of both the power control assembly 4 andthe power monitoring (photodiode) devices.

Due to the short duration between pulse impact on the ball bond and thearrival of the surface wave to the detector (normally a 200 nanosecondwindow), synchronization of the data capture timing becomes an importantpart of the overall system design. Traditional computer control provedto be too slow and ineffective in synchronizing the time between lasertriggering and start of data capturing. It is inherent in most pulselasers that there is a variable delay between the time it receives acommand to fire (trigger) and the time it actually fires. The timevariance is normally in thousandths of a millisecond (0.001 seconds).The same order of magnitude of timing variation can be found in theresponse of a computer executing a laser triggering or data capturingcommand. Due to the close proximity between the pulse and detector beamson the test surface (approx. 0.005-0.010″), the useful surface waveduration is in the order of 200 nanoseconds (0.0000002 second). Asillustrated in the timing chart of FIG. 7, inherent delays might easilycause data capture to be missed and make it difficult to determinewhether the waveform observed is the waveform of interest from the ballbond, or a preceding surface wave.

This synchronization/power control design serves several purposes. Themost important benefit from this design is the synchronization of datacapture to the application of the pulse laser. As photodiode 606 sensesa predetermined rise in voltage, which is a good indication that thepulse laser has fired and that its pulse is on its way to the top of theball bond or interconnect, the data capture window is immediatelyactivated or synchronized by the Analog to Digital (A/D) converter. Thisapproach compensates for delay time in computer processing as well asinherent laser trigger delays.

In addition, this synchronization technique assures a consistentstarting point for signal monitoring, capturing and averaging. Thissynchronization technique is important for supporting subsequent signalaveraging methods for improving signal to noise ratio. A slight shift intime zero will null the surface wave propagation signals (using signalaveraging methods), resulting in a much weaker reported signature thanactual. Further, this technique provides a quantitative measurement ofthe output power (in millivolts) as the laser pulse travels through itspath. This output power can be used to monitor the performance of thepulse laser.

As another feature, the amplitude of the output power can also be usedto calibrate and normalize signature amplitude as part of the signatureanalysis equation.

It was observed that prior art attempts at laser bond integritydetection were primarily directed to larger objects such as a solderjoint. The distance between the pulse and detector is further apartcompared to micro-interconnects. The arrival time and signature durationis much longer as well. On the micro scale, the present inventionoperates with a distance between the pulse and detector laser spotswhich is much closer, resulting in a shorter arrival time. Therefore thesynchronization requirement for the present invention is much moresignificant than in prior art systems.

Referring again to FIG. 2, a commercial off-the-shelf color camera 6 isused for targeting and viewing of the material under test. The in-linelaser reflectors, focus lens and objective allows the viewing andfocusing of the camera, the pulse and detector lasers onto the materialbeing tested. A density filter can be used to filter out unnecessaryglare from both the detector and pulse lasers. This video camera isconnected to a vision system for determining the target locations.

A common objective lens 7 magnifies the field of view for the pulselaser, detector, and video camera. Since the materials being tested aresmall (balls, wedges and interconnects), the field of view has to bemagnified. Because of the dimensions involved, it is difficult toprovide the pulse laser, detector and video camera with their ownrespective optical paths, so in the preferred embodiment a commonobjective lens accommodates all three systems.

The reflection of detector laser 8 is used to monitor vibrationspropagating through the substrate following the application of the pulselaser. The detector laser is preferably a stabilized, single frequencylaser, with continuous wave (CW) of 1064 wavelength. Since the bondintegrity information is imbedded as part of the return (reflected)beam, the amplitude is, therefore, dependent on the reflectivity of thematerial surface and the power level of the detector laser. Preferably,the detector laser power may be set at approximately 700 milliwatt tooptimize the signal-to-noise ratio. The inventor has tested the systemusing lower detector power levels (i.e. 15, 40 and 500 milliwattrespectively), but with limited success. The advantage of high detectorlaser power is a higher amplitude return signal, but additional laserpower may also deposit excessive heat to the test surface. A balancebetween these competing concerns must be struck based on the nature ofthe materials under test.

Beam expander 9 controls the collimation or divergence of the laser beamas it exits detector laser 8. This beam expander is necessary to controlthe beam (spot) size of the pulse laser when it reaches the point ofinterest (i.e. the surface of the test material). As for the spot sizeof the detector, will provide best results when the beam is small andfocused. However, the high power density vs. signal amplitude, discussedabove must be considered.

A set of wave plates and polarized beam splitters 10, 11, 12, 13, and 14are used to control the amplitude and flow of the laser beam from exitof detector laser 8 to the return beam at photodiode 18. The arrangementof the wave plates and the polarization of the laser beam as it travelsthrough the system is illustrated in more detail in FIG. 8.

As the laser beam exits detector laser 8 (shown in FIG. 2), it travelsthrough the first half wave plate 10 which polarizes the beam tohorizontal orientation. Polarizing beam splitter 11 reflects thehorizontally polarized beam towards the quarter wave plate. As the beamtravels through the ¼ wave plate, it is reflected onto test surface 21through objective lens 7 (shown in FIG. 2) by laser reflector 20. Thebeam is then reflected from the test surface 21 and laser reflector 20back through quarter wave plate 12. At this time the polarization of thebeam is rotated 90° from its entry orientation, to verticalpolarization. The vertically polarized beam will travel through beamsplitters 11 and 13 and quarter wave plate 11. As the beam returns frominterferometer 15, quarter wave plate 11 will rotate the polarization ofthe beam 90° from its entry orientation, in this case, to horizontal.The beam is then reflected from polarizing beam splitter 13 throughfocus lens 17 (shown in FIG. 2) onto photo detector 18.

Focus lens 17 focuses the beam onto the center of photo detector 18.Besides manipulating the polarity of the beam, half wave plate 10 isalso used to control the power level of the detector laser beam prior toentering the test sample. Only the horizontally polarized portion of thebeam is reflected to the test surface while the vertically polarizedportion of the beam will go through polarized beam splitter 11 and iscaptured by a laser trap for safety reasons. Rotating half wave plate 10will divert desired portions of the beam to the appropriate direction.

A Fabry-Perot interferometer 15 is used in the preferred embodiment,with 93% reflectors used inside the interferometer. Selection of thesereflectors will provide analog responses from the detected surfacedisplacements. Other interferometers having similar performance, such asHomodyne interferometer, can also be used for this application.

Referring again to FIG. 1, normally the ball bonds, wedge bonds andinterconnects are pulsed by the pulse laser and the detection isperformed at the neighboring surface. It has been found that it ispossible to pulse the material on one side of the ball bond and detecton the same surface on the opposite side of the ball bond. FIGS. 9a and9 b illustrate this alternative test technique in which pulsing anddetecting occur on the same material surface and on the opposite side ofthe interconnect. The resultant signature differs from the firstembodiment discussed above (i.e. pulsing the ball and detecting on thesurface). Instead of looking for a positive wave form caused by thejoining surfaces (intermetallics), this technique detects the dampeningeffect caused by the bonded mass of the ball bond, wedge bond or theinterconnect.

As illustrated in FIG. 9a, a surface wave will be detected without thepresence of a ball bond. On the other hand, when a ball bond is situatedbetween the pulse and detector, a dampening effect, or a weakersignature can be detected as shown in FIG. 9b. Depending on the bondintegrity of the ball bonds, a different degree of amplitude and waveform is experienced. For a well bonded ball, the surface wave isdiminished. For a non-attached but contact bond, the surface wave stayedintact or undisturbed. By correlating the change of amplitude and waveforms, varying degree of bonding for interconnects (ball bonds, wedgebonds, solder bonds and interconnects) can be determined at high speed.

This alternative embodiment has many advantages over previously knowntest methods. First, the pulse laser need not contact the interconnectother than its neighboring surface. This is especially important wherethe interconnect is extremely fragile or sensitive to heat generated bythe pulse laser. Also, the arrival time is much faster than previoustechniques because the surface propagation travels only over the surfacebetween the pulse and detector laser beams. It does not have to travelfrom the top of the ball bond to the surface of the substrate. Dependingon the size of the ball, the travel time ranges from 55 to 70nanoseconds.

The most important benefit of this technique is that the wave form ismuch simpler for correlation. In this case, the waveform is not affectedby the shape of the bond wire protruding from the top of the ball bond.The resultant waveform normally takes the general form of the surfacewave. Also, the spot size of the pulse laser is not limited by the sizeof the ball bond, wedge bond or other interconnect.

This technique is most effective when the detector beam is positionedclose to the interconnect. The pulse laser, however, does not have to beclose to the interconnect for effective results. One caution in usingthis technique is that, in the event that the surface material issensitive to the heat generated by the pulse laser, a larger pulse spot(such as line shape pulse laser) should be used to prevent materialablation. A larger pulse spot with the same power setting will reducepower density on the surface of the test materials, thus eliminatingablation effects.

Concentric detection techniques which are also useful in someembodiments of the present invention will now be described in moredetail.

Due to the sensitivity of the integrity detection according to theinvention, it has been found that the surface wave propagationcorresponds directly to the structural configuration of the joiningintermetallic. Since most joining intermetallics are not perfectlyuniform, the surface wave propagations radiating from the center of aball bond are not uniformly concentric either. By capturing multiplesignatures from different positions around the ball bond, sequentiallyor concurrently, the structure of the joining intermetallic can bededuced. FIG. 10 illustrates the pulsing and detection technique fordetermining the intermetallic structure using laser ultrasonictechniques. The ball 108 is pulsed from the top and detection isperformed on the joining surface, 360° around the ball at a plurality oflocations 1002. To avoid the need for a plurality of detection lasers,optics, and data processing systems, a single detection apparatus may beused sequentially to obtain waveform data for each of the plurality oflocations 1002. For example, a ball bond is placed on the concentriccenter of a rotary table prior to initiating testing. A detector beam ispositioned at a predetermined distance from the center of the ball, say0.006″. A pulse (or a series of pulses) is directed onto the top of theball bond, radiating surface waves outwardly from the center of theball. The detector picks up the surface waves and is printed or stored.The ball bond is then rotated by turning the rotary table at apredetermined angle, say 5 degrees. A second pulse or series of pulsesare initiated and the signature(s) detected and stored. The process isrepeated until signatures at all points 1002 around the ball bond arecollected. The distance between detector points depends on the radialresolution desired.

Once these signatures are collected, they can be displayed in 3-Dformat, thus providing the propagation “view” of the intermetallicstructure. This format can be correlated or interpreted to the structureof the joint and displayed for viewing.

The laser trigger control in the present invention is part of a systemcontrol design which monitors the status (sensitivity positions) of theBond Integrity Tester, and initiates the triggering of the laser (orinitiates the test). This trigger control compensates for the inherentwavelength (λ) drift of the detector laser beam as well as the mirrorpositions inside the interferometer. System performance and sensitivityto surface propagations are directly affected by these driftingproblems. In other words, if tests are performed at the precise optimalsystem sensitivity, strong signatures will result for analysis. On theother hand, if the tests were performed when the wavelength shiftedslightly or the mirror positions have drifted off its pre-set positions,signatures received will be weak or even non-detectable. These driftingeffects are amplified when signal averaging techniques are employed. Toovercome the above mentioned problems and assure consistent testresults, two techniques have been developed, which will be referenced asthe Static and Dynamic Triggering Control Modes, respectively.

The static control trigger mode is used primarily for manual testingwhere the user can view and set the laser trigger point prior to eachtest. As illustrated in FIG. 11, a circuit monitors the amplitude(voltage output) of photodetector 16 from the interferometer 15 andprovides feedback through an analog oscilloscope 1102. This feedbackprovides a reference between the laser triggering point and the actualoptimal sensitivity of the system. The user has the option to re-set thetrigger point at any sensitivity level of the system (i.e. mostsensitive, least sensitive, or anywhere in between) prior to triggeringthe laser. This setting is accomplished by adjusting the distancebetween the reflective mirrors inside the interferometer cavity toaccommodate the drifting effect discussed above. Operation of thisStatic Trigger Control Mode is accomplished by adjusting a knob at theinterferometer mirror controller 1104 while observing the photodiodefeedback. Once the trigger point is set, the user can initiate thefiring of the trigger. This mode is especially useful for single pulsetesting and system calibration. The drawback of this mode is thataccommodation for drift is manual and therefore more time consuming.However, the user has the freedom to set the laser trigger pointanywhere along the sensitivity range of the system, for investigativeand calibration purposes.

The dynamic trigger control mode is optimized for automatic testing athigh speed with simultaneous adjustment to sensitivity drift. In thismode, the mirror inside the interferometer is set at a continuous scan(oscillating) mode such that multiple photo peaks can be observed (i.e.the interferometer mirror is made to travel a distance of severalwavelengths of the detector laser frequency and return). For example,the distance may be set at 3 photo peaks—regardless of frequency drift,the system will cross over six (6) optimal sensitivity points (3 one wayand 3 return). As illustrated in FIG. 12, a circuit monitors thereturned peaks (blanking periods) and especially the first (1^(st)) peakafter the blanking period. The circuit also has a comparator circuit tocompare the voltage amplitude from the 1^(st) photo peak (as it isscanned) against a pre-determined voltage value stored in its PROM. Oncethese two voltage values match, the circuit initiates a laser triggercommand to the laser. The voltage value stored in the PROM is preferablya value that will be achieved at time Tf before the optimal photo peak.,where Tf is an average time between transmission of a signal to fire thepulse laser, and the actual firing of the pulse laser.

The Dynamic Trigger Control Mode circuit performs continuous triggeringand keeps track of the number of triggers for each session. Once thetrigger command equals the trigger count value stored in the PROM, thetrigger session ends. Both the trigger count value and thepre-determined voltage amplitude in the PROM are input from the systemcontrol computer prior to each test session.

Since the interferometer control system of the present inventionsupports a scanning rate of 100+ cycles per second, testing of joints atoptimal sensitivity can, therefore, be performed in excess of 100 pointsper second as well. This triggering control technique supports highspeed testing with automatic accommodation to sensitivity drifts in thesystem.

The Dynamic Trigger Control Mode has many advantages. This mode adaptsto inherent sensitivity drifts in the system automatically, supportscomputer controlled automatic testing (PROM inputs for count &sensitivity criteria), assures integrity tests at optimal systemsensitivity (or any other pre-set sensitivity) from the PROM, supportshigh speed testing, and tracks single or multiple tests automatically.

A number of techniques are used in the present invention to improvesignal to noise ratios. Signal-to-noise has always been a challenge toall electronics systems. This Non-Contact Bond Integrity Technology isno exception. In order to acquire the minute surface propagationsgenerated at the bond, our detector sensor must be extremelysensitivity. Beside acquiring the surface propagation waves, otherenvironmental noise such as Q-switch RF noise and ground noise are beingpicked up as well. The amplitude of these noises, at times, are greaterthan those of the propagation signatures. Since most environmental noiseis random, while our surface wave is fixed, a signal averaging techniqueis used to cancel out these unwanted noises.

Beside signal averaging techniques, other considerations for increasingthe amplitude of the signatures are desirable. The most significantimplementation issues are careful selection of appropriate photodiodes,increased detector laser power, and reduced detector laser spot size.

Photodiodes that were supplied by the interferometer manufacturers usedsilicon based photodiodes. It was found that system operation isimproved by substituting photodiodes that are more responsive to thepreferred detector laser frequency of A λ=1064 nm. As illustrated inFIG. 13, gallium arsenide photodiodes are approximately 4.3 times moreresponsive to the preferred detector laser frequency. This equates to inexcess of a 400% improvement in signal-to-noise ratio.

The amplitude of the return signature is dependent on the amount oflight reflected from the test surfaces. Due to the tremendous loss oflight energy through the optics layers, the returned light energy issubstantially attenuated. An increase of input detector laser power from15 mW (as disclosed in the inventor's prior patent) to 700 mW wasimplemented and the signal-to-noise ratio was increased many times.

It was also found that minimizing the spot size of the detector laserincreased the signature amplitude. Detector spot size was reduced to0.003″ in diameter for an improved signal-to-noise ratio.

Data collected using the present invention has substantiated thatintegrity of micro dimensioned material bonds can be determined usingnon-contact laser ultrasonic means. A surface wave signature generatedby the above process is captured by an Analog to Digital converteroperating at 500 MHz (i.e. 500 million samples per second). Preferably,500 data points are collected for each signature at 2 nanoseconds perpoint. FIG. 14 shows signatures for a well bonded ball bond, a partiallybonded ball and a non-bonded (touching) ball. Note that the design ofthe present invention eliminates shock waves that were present in thesignatures in the inventor's prior U.S. Pat. No. 5,535,006.

In the preferred embodiment, only the early segment of the signature isused for correlation because surface waves rebounding from itsneighboring bonds (structures) may arrive at the detection point, thusresulting in a complex signature. Besides amplitude of the spikes,several features of the signature are used for bond integritycorrelation. The combination of these features (seed parameters)provides a much better distinction between a good, partial and non-bond.FIG. 15 shows a sample signature which will now be referenced inexplaining elements of the signature analysis used in the presentinvention.

The arrival time of the first surface wave spike (Point C in FIG. 15) isdetected to assure that the system is performing as expected because thesurface wave, regardless of its bond integrity, should travel at thesame velocity. Abnormal arrival time normally indicates unusual bondstructures or interconnect features. The arrival time of a surface waveis dependent on the materials being used, the Rayleigh velocity and thedistance between the pulse and detector laser spots.

The vertical separation between Pt. C and Pt. E as shown in FIG. 15increases as the shear force increases (or as the bond integrityimproves) calculating the vertical distance between the two points. Thedistance is an indicator of bond integrity and is used as one of theseed parameters for predicting bond status of the interconnect.

The slope between Pt. D and Pt. E correlates closely with shearstrength. Our data analysis software routine identifies the Maximumpoint (Pt. D) and Minimum point (Pt. E) within a pre-determined timewindow automatically and performs a regression analysis with all thedata points in between. The slope of this regression analysiscorresponds directly to the bond integrity of the interconnect. Thesteeper the slope, the stronger the bond is experienced. Conversely, asthe slope decreases, the weaker the bond forces.

Frequency, in this application, can be translated into arrival time ofpeaks and valleys of the 2^(nd) and 3^(rd) spikes. Among the 4 points(F, G, H and I), arrival time of Pt. I was used as a preliminary seedparameter. It is because Pt. I is the furthest point out in time andallows for better time separation among signatures of varying degrees ofbonding. Empirical test data supports the above analysis.

FIG. 16 is a schematic diagram of a preferred embodiment of a completeinspection system according to the present invention. The system 1600comprises system controller 1602, vision system 1603, ring illumination1604, detector laser subsystem 1605, laser steering motors 1606,interferometer control circuit 1607, amplifier 1608, pulse lasersubsystem 1609, synchronization control 1610, digital scope transientrecorder 1611, photodiode 1613, analog scope 1614, input devices 1615,motor control 1618, interferometer 15, photodiode 16, and video camera6. Pulse laser subsystem 1609 and detector laser subsystem 1605 arepreferably constructed according to the description above andparticularly with reference to FIG. 2. It will be apparent to thoseskilled in the art that much of the apparatus of FIG. 2 has been omittedfrom FIG. 16 for clarity, but that the same elements are preferablyincorporated in the complete system of FIG. 16.

A functional flowchart showing a preferred embodiment of an automatedtest process is provided in FIG. 17. To start the test process, as shownin Block 1702, the operator places the part to be tested on the table21, which in this embodiment is equipped with motors to translate italong two axes to form a transport stage. Next, in Block 1704, systemcontroller 1602 moves the part under the vision system 1603 whichcontrols part alignment and positioning. Then in Block 1706, the systemcontroller energizes the ring illumination mechanisms 1604 to highlightthe ball bonds and wedge bonds within the field of view. The ringillumination mechanisms and additional operating features of the systemare preferably constructed according to the disclosures of theinventor's prior U.S. Pat. Nos. 5,420,689; 5,424,838; and 5,302,836, thedisclosures of which are incorporated herein by reference. In block1708, vision system 1603 identifies and locates the ball bonds, wedgebonds or interconnects to be inspected, and in Block 1710 it sendstarget coordinates to system controller 1602. System controller 1602performs positioning modification using the transport stage 21 for thedetector laser subsystem 1605 as well as the pulse laser steeringmirrors 1606 for pulse laser targeting, as shown in Block 1712. Systemcontroller 1602 then sends a command to interferometer control circuit1607 to perform the integrity test. The command, consisting of number ofpulses and sensitivity level set point for laser triggering, are storedin the PROM of the ICC 1607.

Next, in Block 1714, ICC 1607 monitors the system sensitivity status viathe amplifier 1608 and triggers the pulse laser subsystem 1609 at anappropriate time for predicted maximum sensitivity, given the expectedtime lag between transmitting the signal and firing of the laser. Oncetriggered, the laser pulse passes through the synchronization controlunit 1610 which in turn, initiates the data capture function of thetransient recorder 1611, as shown in Block 1716. The surface wavesignature is captured and converted to voltage via the Interferometer 15and photodiode 16 in Block 1718 and the signature is recorded by thehigh speed transient recorder 1611 for display. Subsequently, this datais also sent to the system controller 1602 for automated bond integrityanalysis (see Block 1720). Results of the test are then displayed (Block1722). Results are preferably stored for further review, analysis, andcorrelation (Block 1724). Depending on requirements for the automatedtest, control passes either to Block 1726 (where the next part is to beinspected), Block 1728 (where the part must be moved to test the nextinterconnect), or Block 1730 (where the test is complete).

The analog scope 1614 is provided for visual reference when static modetrigger operations are performed (as described above). The inputelements 1615 may include a joystick, keyboard and mouse to enhanceusability of the operating software.

In a preferred embodiment, the two oscilloscopes are implementedvirtually in a single computer screen display, along with live videofrom video camera 6 for targeting purposes, on-screen mouse driven focuscontrols for video camera 6, and on-screen mouse driven controls forrotating and translating the transport stage.

In addition to evaluation the integrity of small bond areas such as ballbonds, the techniques and systems disclosed herein can be used to testbonding integrity in various other micro level applications. Inparticular, the invention can be used to test bond integrity in largerareas such as in thin coating. A good example of this application isdetection of the presence and absence of voids, non-bonds but incontact, partial bond and fully bond conditions. Due to the thin coatingof the material (on the order of microns), power density may be toostrong if a spot pulse laser is applied. To overcome this potentialproblem, the preferred embodiment of this application uses analternative optics design which decreases power density on the surface,yet maintains a high signal-to-noise ratio. Signature characterizationrelating to thin coating was addressed. A mathematical model isdeveloped for each application (for example, a model for evaluating thinaluminium coating on silicon and correlating to actual signaturesacquired by the system is described in more detail below).

Ablation may occur due to high density of energy being focused on thesurface to create the propagation or surface wave. The inventor hasdetermined that, power intensity of the pulse is directly proportionalto the amplitude of the signature. In other words, the higher the pulseintensity, the stronger the pulse—thus a more recognizable signature. Onthe other hand, to reduce ablation effects, the lower the intensity ofthe pulse, the weaker the signature—thus decreasing the signal-to-noiseratio.

A significant consideration in system design is maintaining thenon-destructive nature of the sensor. If pulse laser beam is focused ina small spot, a large amount of energy (Mwatts) can be deposited ontothe sample surface in a short time (nsec), causing material ablation. Bymodifying the optics design, an elongated pulse laser spot was providedfor this embodiment. The elongated laser foot print reduces the powerdensity on the surface materials, thus allowing a higher total inputpower to the surface, while keeping the surface temperature belowablation level. This new design provided two benefits to our bond testapplications. First, it allows the testing of ultra-thin materials withno ablation. Samples under test were validated and documented under aScanning Electronic Microscope (SEM) under 600× and 950× magnificationsrespectively. In addition, mathematical models using heat transfertheories, were developed to determine the surface and sub-surfacetemperatures. Both methods validated our non-ablation achievement.Second, this optics design overcame the problem of power dispersion froma single spot energy source. As illustrated in FIG. 18, energy dispersesin all directions from a single point source. With a single detectionpoint positioned at some distance from the source, a theoretical 1/360power is received for every degree of dispersion. For a detectablesignature, high energy must be applied onto the surface, if a singlepoint source is used. As shown in FIG. 19, the line laser sourcerecombines the dispersed energy at the detection point. This approachprovides a highly detectable signature at low power density.

The optics design utilizes a succession of cylinder lenses to create anintermediate line image ahead of the objective lens. In the preferredembodiment, the line shaped beams are 1 to 2 mm long. The beam size canbe changed by varying the spacing between the negative cylinder and thefirst positive cylinder. The optics design is based on Code V raytracing software. This optical system uses a succession of cylinderlenses to create an immediate line image. By varying the focal length ofthe first lens in the system (the negative cylinder), the F/no. of theimmediate image can be varied which in turn controls the width of thebeam in the focus of the objective lens. The optics design is shown inAppendix A.

The adherence of coatings on material surfaces is an important problemin many manufacturing settings from automobile to microelectronicscircuit manufacturing. Usually these coatings are thin (1-40 microns),and deposited on base materials which have very different optical,chemical, thermal or mechanical properties.

The laser ultrasonic approach of the present invention measures thecharacteristics of elastic wave propagation within a region thatencompasses both the coating and the base material. This is a totallymechanical measurement dependent on mechanical properties, such ascoating thickness, elastic constants and the condition of the bondbetween the coating and its base material. Elastic waves consist ofelastic strains that can propagate considerable distances along thecoating surface. These strains stretch the coating/substrate interfacethereby testing the bond strength directly. Strains both parallel andperpendicular to the interface plane can be applied either together orindividually through the use of polarized elastic wave modes on thecoating and its base material.

Along the surface of a material, the travelling elastic wave mode isknown as a Rayleigh wave. It is much like the waves that propagate onthe surface of liquids, such as waves in the ocean, except that therestoring force is elastic rather than surface tension. This wavetravels for long distances and does not change its shape, only itsamplitude due to attenuation. The speed of this wave depends on theelastic constants of the material.

If the coating material is a thin sheet, two surface waves will travelalong the material, one on the top surface and the other on the bottomsurface. These waves will propagate as surface waves, as illustrated inFIG. 20. This phenomenon can be explained as follows.

As the pulse laser impacts the surface 2001, waves with varyingfrequencies are generated along the surface 2001 of the coatingmaterial. The propagations with wave lengths shorter than the thicknessof the coating material stay on top of the coating as shown at 2002 andpropagate outwards at the Rayleigh velocity of the coating material. Onthe other hand, the wavelengths longer than the thickness of the coatingmaterial will penetrate through the coating materials onto the basematerial 2004. Since the base material is normally thicker than thepenetrating wave length, these wave frequencies 2006 will travel alongthe top of the base material at the base material Rayleigh velocity, tothe detection point. When this situation occurs, two surface waves 2002and 2006 can be detected—one arrives at a different speed than theother. Using Fast Fourier Transform (FFT) analysis of these signatures,and correlating to its wavelengths, the thickness of the coatingmaterial can be readily determined.

As discussed above, the wave dispersion behaviors make identification ofthese modes very easy. In addition, the degree of dispersion of thesewaves are dependent on the thickness of the materials. FIG. 21 shows theRayleigh wave velocity of a surface wave plotted against the frequencyof the wave, when travelling along an aluminum coated silicon material.As indicated, the velocity of the propagation wave slows down as itsfrequency increases. If one monitors the arrival time of a highfrequency component of the wave, say 100 MHz, one can determine thethickness of the coating material. Alternately, the shape of thesignature dispersion detected can also be used to determine thethickness of the coating. Sample wave forms in Appendix B illustratedispersed wave forms on various thickness of aluminum coated silicon.The samples show the wave form at separation distance of 20.0 mils forcoating thickness of 1.0 μm, 5.0 μm, 10.0 μm and 20.0 μm respectively. Avery different wave form can be detected due to different coatingthickness.

Adhesion characteristics of the joining materials can be determined bythe disturbance or out of normal asymmetric waveform. For example, ifthere is total non-adhesion between the coating and base materials, aflat waveform will be detected.

In another preferred embodiment of the thin coat testing method, a newlaser beam design was made to further improve the signal-to-noise ratio.Instead of a spot or a straight line footprint. A curved line pulselaser foot print was developed. Referring now to FIG. 22, the curvedlaser 2202 allows the same or better power density on the surface of thematerials, the surface wave propagation will be focused to a singledetection point 2204. When positioning the detection beam at the focalpoint of the curved laser beam 2202, the amplitudes of the signaturesare summed, thus increasing the signature amplitude many times. Thistechnique is especially useful when the material layers are sensitive toheat induced by the pulse laser. One important criteria for using thistechnique is that the detector laser foot print must be accuratelypositioned at the focal point of the curved laser beam. The disadvantagein using this technique is that the curvature of the laser beam has tobe changed accordingly when the distance between the pulse and detectorlaser is changed.

The various embodiments of the invention may incorporate any or all ofthe features disclosed in the inventor's prior U.S. Pat. No. 5,535,006,U.S. provisional App. No. 60/068,362 filed Dec. 19, 1997, and/or U.S.non-provisional app. Ser. No. 09/215,374 filed Dec. 18, 1998, thedisclosures of which are all incorporated herein by reference.

Thus, there has been disclosed an improved system and method formonitoring interconnections between elements. The invention is notlimited to the specific examples disclosed herein, but includes allvariations encompassed by the language of the claims which follow.

What is claimed is:
 1. A laser ultrasonic system for evaluating a bondbetween an element and a substrate, comprising: Pulse application meansincluding a collimated light source for transmitting a pulse of lightalong an optical path to a target point on a substrate; Firing detectionmeans in said optical path for detecting the passage of said pulse oflight along said optical path and providing an output signal indicatingthe presence of the pulse; Monitoring means for detecting thepropagation of vibrations in said substrate, and collecting for analysisa vibration signature reflecting said propagation of said vibrations;Synchronization control means connected to said pulse application meansand said firing detection means for selectively actuating said pulseapplication means to provide a light pulse to said target point,monitoring said firing detection means to determine the timing of saidpulse transmission, and in response thereto, activating said monitoringmeans to collect a pertinent part of said vibration signature relativeto said timing of said pulse transmission.
 2. The laser ultrasonicsystem in accordance with claim 1, wherein said element comprises acoating bonded to said substrate.
 3. The laser ultrasonic system inaccordance with claim 1, wherein said element comprises an element fromthe group consisting of ball bonds, wedge bonds, circuit traces, ribbonbonds, welds, solder balls, surface mount components, pin grid arrays,MIMMS, and adhesion media.
 4. The laser ultrasonic system in accordancewith claim 2, further comprising: means for analyzing said vibrationsignature to determine the degree of integrity of said bond between saidcoating and said substrate.
 5. The laser ultrasonic system in accordancewith claim 4, wherein said means for analyzing comprises means fordetecting the presence or absence of a void between said coating andsaid substrate.
 6. The laser ultrasonic system in accordance with claim4, wherein said means for analyzing comprises means for detecting thepresence or absence of a crack in at least one of said coating and saidsubstrate.
 7. The laser ultrasonic system in accordance with claim 4,wherein said means for analyzing comprises means for detecting anon-bond and non-contact condition between said coating and saidsubstrate.
 8. The laser ultrasonic system in accordance with claim 4,wherein said means for analyzing comprises means for detecting anon-bond-but-in-contact condition between said coating and saidsubstrate.
 9. The laser ultrasonic system in accordance with claim 4,wherein said means for analyzing comprises means for detecting a partialbond or full bond condition between said coating and said substrate. 10.The laser ultrasonic system in accordance with claim 2, wherein saidcoating comprises a thin film coating.
 11. The laser ultrasonic systemin accordance with claim 10, wherein said thin film coating comprises ametallized thin film coating and wherein said substrate comprises asemiconductor-based substrate.
 12. The laser ultrasonic system inaccordance with claim 11, wherein said metallized thin film coatingcomprises an aluminum thin film coating.
 13. The laser ultrasonic systemin accordance with claim 11, wherein said metallized thin film coatingcomprises a gold thin film coating.
 14. The laser ultrasonic system inaccordance with claim 11, wherein said semiconductor-based substratecomprises a silicon-based substrate.
 15. The laser ultrasonic system inaccordance with claim 11, wherein said semiconductor-based substratecomprises a gallium arsenide-based substrate.
 16. The laser ultrasonicsystem in accordance with claim 2, wherein said target point is a pointon said coating.
 17. The laser ultrasonic system in accordance withclaim 2, further comprising an optical subsystem deployed between saidcollimated light source and said target point.
 18. The laser ultrasonicsystem in accordance with claim 17, wherein said optical subsystemcomprises means for shaping said pulse of light to form an elongatedpulse laser spot at said target point, whereby a reduced power densityis delivered to said target point.
 19. The laser ultrasonic system inaccordance with claim 1, further comprising an optical subsystemdeployed between said collimated light source and said target point,said optical subsystem controlling the spot size of a detector laserbeam associated with said monitoring means.
 20. A method for evaluatingthe integrity of a bond between a substrate and a coating, comprisingthe steps of: Using a collimated light source to apply a pulse of lightto a target point on a coated substrate; Detecting transmission of saidpulse along an optical path between said collimated light source andsaid target point, and providing an output signal indicating thepresence of said pulse; Detecting propagation of vibrations in saidsubstrate, and collecting for analysis a vibration signature reflectingsaid propagation of said vibrations; and, Determining timing of saidpulse transmission, and in response thereto, activating monitoring meansto enable analysis of a pertinent part of said vibration signaturerelative to said timing of said pulse transmission.
 21. The method inaccordance with claim 20, further comprising the steps of: analyzingsaid vibration signature to determine the integrity of said bond betweensaid substrate and said coating.
 22. The method in accordance with claim21, wherein said analyzing step comprises a step of applying afrequency-domain analysis to said signature to determine the thicknessof said coating.
 23. The method in accordance with claim 21, whereinsaid analyzing step comprises a step of applying a time-domain analysisto said signature to determine the thickness of said coating.
 24. Amethod for evaluating the integrity of a bond between a coating and asubstrate, comprising the steps of: Using a continuous wave laser andinterferometer to produce an interference pattern from light reflectedby said coating or said substrate; Using a detecting means to monitorchanges in said interference pattern over time; Selecting a preset pointof optimum system sensitivity along the duration of said changes;Triggering a collimated light source to apply a pulse to a target pointon said coating or said substrate when said selected preset point isreached; and, Using said detecting means to detect the propagation ofvibrations in said substrate resulting from said pulse, and collectingfor analysis a vibration signature reflecting said propagation of saidvibrations.
 25. The method in accordance with claim 24, wherein saidstep of applying a pulse to a target point comprises the step ofapplying a pulse having a curved line foot print.
 26. A laser ultrasonicsystem for evaluating a bond between an element and a substrate,comprising: Pulse application means including a collimated light sourcefor applying a pulse of light along an optical path to a target point;Firing detection means in said optical path for detecting the passage ofsaid pulse of light along said optical path and providing an outputsignal indicating the presence of the pulse; Monitoring means fordetecting the propagation of vibrations in said substrate, andcollecting for analysis a vibration signature reflecting saidpropagation of said vibrations; Synchronization control means connectedto said pulse application means and said firing detection means forselectively actuating said pulse application means to provide a lightpulse to said target point, monitoring said firing detection means todetermine the timing of said pulse transmission, and in responsethereto, activating said monitoring means to collect a pertinent part ofsaid vibration signature relative to said timing of said pulsetransmission.
 27. The laser ultrasonic system in accordance with claim26, wherein said element comprises a coating bonded to said substrate.28. A laser ultrasonic system for evaluating a bond between an elementand a substrate, comprising: Pulse application means including acollimated light source for applying a pulse of light along an opticalpath to a target point; Monitoring means for detecting the propagationof vibrations in said substrate, said monitoring means comprising adetection apparatus for collecting a plurality of vibration signaturesreflecting said propagation of said vibrations from a plurality oflocations around said element; means for using said plurality ofvibration signatures to create a three-dimensional propagation view ofsaid bond between said element and said substrate.
 29. The laserultrasonic system in accordance with claim 28, wherein said detectionapparatus comprises a plurality of detectors positioned at saidplurality of locations.
 30. The laser ultrasonic system in accordancewith claim 28, wherein said detection apparatus comprises a singlesensor used sequentially to obtain waveform data for each of saidplurality of locations.
 31. The laser ultrasonic system in accordancewith claim 28, wherein said three-dimensional propagation view iscorrelated or interpreted to the structure of said bond and displayedfor viewing.
 32. A method for evaluating the integrity of a bond betweenan element and a substrate, comprising the steps of: Using a continuouswave laser and interferometer to produce an interference pattern fromlight reflected by said element, said bond or said substrate; Triggeringa collimated light source to apply a pulse to a target point on saidelement, said bond or said substrate; Using a detector to detect saidinterference pattern and collect for analysis a vibration signaturereflecting said propagation of said vibrations; and, extractinginformation from said vibration signature to determine the integrity ofsaid bond between said element and said substrate.